Under President Xi Jinping, long-standing caution has been noticeably slipping.

China's foreign relations in modern times—particularly with the U.S. and Asian neighbors—have been guided by a doctrine of prudence embodied in Deng Xiaoping's famous maxim "taoguang, yanghui"—"bide our time; hide our capabilities."

Mr. Deng correctly calculated that once the world understood the full implications of an emerging Asian giant, it would stir up anxieties and upset the external harmony that China needed to pursue its economic development. He was suspicious, too, that the U.S. secretly...